1. Field of the Invention
The present invention relates to an image forming apparatus, such as laser printers and electronic copiers, which perform image formation on recording media using developing agents.
2. Description of the Related Art
Generally, the print factor for text on recording media in image forming apparatus, such as laser printers and electronic copiers, is of the order of 5 [%]. On the other hand, the print factor in the case of printing addresses on envelopes is of the order of 0.5 [%], and this only consumes approximately 1/10 of the developing agent used when printing ordinary text.
FIG. 20 is a diagram showing the relationships between the print factor and the image density and between the percentage occurrence of white background fogging and the print factor when printing was performed by a laser printer using a two-component developing agent. From this diagram, it can be seen that, the lower the print factor, the less the image density and also the greater the white background fogging. When the cause of this was studied, there was found to be a correlation between the mean particle size of the developing agent and the percentage occurrence of white background fogging, as shown in FIG. 21. There was also found to be a correlation between the mean particle size Of the developing agent and the image density. That is, the smaller the mean particle size of the developing agent, the more the image density reduces and the more the white background fogging increases. The relationship between the print factor and the mean particle size of the developing agent is as follows. That is, the fact that the print factor is low means that the renewal of the developing agent within the developing device is poor, and thus the same developing agent stays in the developing device for a long period. Therefore, it is considered that the developing agent is repeatedly crushed by friction with the inner wall of the developing device through the rotation of the developing roller and the agitating rollers so that its particle size becomes smaller.
Moreover, in the case of a one-component developing agent, a reduction Of image density immediately appears when the print factor is reduced In other words, this is because, with a one-component developing agent, the print factor is low, any developing agent which stays on the developing roller without adhering to the photosensitive drum is repeatedly rubbed by the developing agent layer thickness regulating member and, as a result, the charge of the developing agent increases.
In this way, with prior art image forming apparatus, there was a problem in that when the consumption of developing agent was reduced due to continued image formation processing with a low print factor, the mean particle size of the developing agent gradually became smaller. This fact resulted in a reduction of image density and an increase of fogging.
U.S. Pat. No. 4,736,255 (Patented Apr. 5, 1988) shows a recording apparatus. In this recording apparatus, a developing agent image corresponding to an image infomation in accordance with data input through a data controller is formed on a photosensitive body by a laser scanner. The developing agent image is transferred to a paper, and the surface of the photosensitive body after a transfer operation is cleaned by a cleaning device. The recording apparatus comprises a developing agent applying mechanism which applies the developing agent to the surface of the photosensitive body, and which is operative when the laser scanner is not operated by a presecribed time.
However, in this recording apparatus, in order to operate the developing agent applying mechanism, this apparatus has not a calculating means to obtain a print factor expressing a ratio obtained by dividing area of developed image formed on the photosensitive body by printable area of the paper.